Radio Frequency Identification (RFID) technology employs a radio frequency (“RF”) wireless link and ultra-small embedded computer circuitry. RFID technology allows physical objects to be identified and tracked via these wireless “tags”. It functions like a bar code that communicates to the reader automatically without requiring manual line-of-sight scanning or singulation of the objects. RFID promises to radically transform the retail, pharmaceutical, military, and transportation industries.
Several advantages of RFID technology are summarized in Table 1:
TABLE 1Identification without visual contactAble to read/writeAble to store information in tagInformation can be renewed anytimeUnique item identificationCan withstand harsh environmentReusableHigh Flexibility/Value
As shown in FIG. 1, a basic RFID system 100 includes several tags 102, a reader 104, and an optional server 106. Each tag 102 includes an integrated circuit (IC) chip and an antenna. The IC chip includes a digital decoder needed to execute the computer commands the tag 102 receives from the tag reader 104. The IC chip also includes a power supply circuit to extract and regulate power from the RF reader; a detector to decode signals from the reader; a back-scattering modulator to send data back to the reader; anti-collision protocol circuits; and at least enough memory to store its EPC code.
Communication begins with a reader 104 sending out signals to find the tag 102. When the radio wave hits the tag 102 and the tag 102 recognizes the reader's signal, the reader 104 decodes the data programmed into the tag 102. The information can then be passed to a server 106 for processing, storage, and/or propagation to another computing device. By tagging a variety of items, information about the nature and location of goods can be known instantly and automatically.
The system uses reflected or “backscattered” radio frequency (RF) waves to transmit information from the tag 102 to the reader 104. Since passive (Class-1 and Class-2) tags get all of their power from the reader signal, the tags are only powered when in the beam of the reader 104.
The Auto ID Center EPC-Compliant tag classes are set forth below:
Class-1                Identity tags (RF user programmable, maximum range ˜3 m)        
Class-2                Memory tags (8 bits to 128 Mbits programmable at maximum ˜3 m range)        Security & privacy protection        
Class-3                Battery tags (256 bits to 64 Kb)        Self-Powered Backscatter (internal clock, sensor interface support)        ˜100 meter range        
Class-4                Active tags        Active transmission (permits tag-speaks-first operating modes)        Up to 30,000 meter range        
In RFID systems where passive receivers (i.e., Class-1 tags) are able to capture enough energy from the transmitted RF to power the device, no batteries are necessary. In systems where distance prevents powering a device in this manner, an alternative power source must be used. For these “alternate” systems (also known as active or semi-passive), batteries are the most common form of power. This greatly increases read range, and the reliability of tag reads, because the tag doesn't need power from the reader. Class-3 tags only need a 10 mV signal from the reader in comparison to the 500 mV that a Class-1 tag needs to operate. This 2,500:1 reduction in power requirement permits Class-3 tags to operate out to a distance of 100 meters or more compared with a Class-1 range of only about 3 meters.
A problem frequently encountered is that of “hot” tags. Because tags communicate with the reader by backscattering the carrier signal, those tags very close to a reader create a very strong backscatter that can interfere with communications between other tags and readers located far away. Two types of interference, or “jamming”, are prevalent: forward link jamming and backscatter jamming. Consider a situation in which passive tag-1 is located 0.5 meters from Reader #1. The communications therebetween include the forward link from the reader to tag, and the backscatter signal from the tag to the reader. The maximum effective range of Reader #1 is 10 m. Passive tag-2 is located 10 meters from Reader #2. Readers #1 and #2 are located 200 meters apart. A “hot” tag-1 located only 0.5 meter from Reader #1 will generate backscatter 400 times greater at 0.5 m than it would at the 10 m maximum range of the reader, as calculated by (max range/actual distance)2=(10/0.5)2=400×. As will soon become apparent, tag-1 generates so much backscatter that it can jam communications between tag-2 and Reader #2, even though both tag-2 and Reader #2 are located 200 meters away from tag-1. In fact, for acceptable communications, Reader #2 would need to be located over 600 meters from tag-1 and/or Reader #1 based on square-law attenuation of RF energy over distance in free space, as calculated by the following equation:D=a×b×(d2/d1)  Equation 1where:
D is the distance between Reader #1 and Reader #2,
a is the extra distance necessary to provide a minimum “tag-2 to tag-1 signal to noise ratio” of at least 10 db (which is a typical minimum ratio that allows tag-2 to successfully communicate with Reader #1) [value can vary depending on system and environmental situation],
b is the maximum effective range of Reader #1,
d2 is the distance between tag-2 and Reader #2, and
d1 is the distance between tag-1 and Reader #1.
Performing the calculation, 3×10×(10/0.5)=600 meters. This is unacceptable in situations where multiple readers may be present in close proximity, as in a shopping mall. In the US, there are about 50 channels available to RFID systems. In Europe, there are currently only 10 channels. Accordingly, as RFID becomes more prevalent, readers will be using the same channels and will be using the same frequency, and the “hot tag” problem will become a serious issue that must be overcome.
Using long-range Class-3 tags and readers makes this “hot tag” problem even worse. For example, a “hot” Class-3 tag-3 located 0.5 m from Reader #3 running at a full 4 Watt (W) Effective Incident Radiated Power (EIRP) power can jam a Class-3 tag-4 located 100 meters from Reader #4 at a range of 60,000 meters in free space, where D=3×100×(100/0.5)=60,000 meters. In English units, this “hot tag” can jam every reader operating in its channel at a range of up to 40 miles away (in free space).
One proposed solution is to have the tag detect its own incident power. If the tag detects a strong signal, it will attenuate its own backscatter. However, this adds complexity and cost to each and every tag, making it cost prohibitive.
What is needed is a cost effective and efficient way to both dramatically reduce the severity of the hot tag backscatter problem and also reduce reader-to-reader interference in the forward link as well.